In the pumping of gases and vapors, such as in the manufacture of semiconductor devices and display screens, it is often necessary to use a high vacuum pumping system. A common pump used for this purpose is a turbo-molecular pump (TMP).
The TMP, used on a wide variety of semiconductor and general applications, relies on a rotating member that rotates near the velocity of the gas molecules to be pumped. A significant feature of such a pump is that the compression ratio of outlet to inlet pressure is very high. Moreover, the exhaust of the TMP, in general, must not be subjected to too high of a pressure. In particular, the differential pressure between the inlet and the outlet of the pump must be kept low.
If the pump is subjected to high pressure, either at the inlet or exhaust, then significant heat and stress are generated within the pump. The heat and pressure can cause the pump to destroy itself. To avoid this situation, the TMP is generally used within a vacuum system that incorporates a bypass line and some control logic to ensure that the pump is operated only when both the inlet and exhaust pressure is initially low.
A typical vacuum system will have a chamber where a process or experiment is to occur, a bypass line with a valve near the inlet to the chamber, a TMP with a valve connected to the exhaust of the TMP, and a valve connected between the inlet of the TMP and the chamber. The exhaust of the TMP, via a valve, is connected to the downstream side of the bypass line.
The bypass line, used in conjunction with valves both on the inlet and the exhaust of the pump, is used to evacuate the chamber to which the TMP is attached. A secondary pump, or backing pump, performs the evacuation. Once the chamber pressure is beneath a certain threshold amount, determined by the design of the TMP, the valve connecting the bypass line to the chamber is closed. Then, the exhaust valve to the TMP is opened and subsequently the valve to the inlet of the TMP is opened. A fluidic connection is now made between the chamber and the backing pump via the inlet valve to the TMP, the TMP itself, and the exhaust valve of the TMP. The TMP now continues to evacuate the chamber.
Due to the intrinsic nature of the TMP vacuum performance, all TMPs require some form of bypass line, bypass valve, inlet valve, and exhaust valve. There are generally some additional gauges connected, for example a pressure gauge or vacuum switch, connected to various points within the vacuum system to monitor its performance and generate signals to external, remote or closely mounted, controllers that actuate the valves.
The aforementioned vacuum systems are assembled from various components, for example, air-actuated solenoid vacuum valves, pipes, vacuum seals, throttle valves, gate valves, TMP, and the like. A key feature of this system is that there is universally an assembly of the various components with multiple seals and connections. Conventionally, there is limited integration of the components.
The aforementioned known vacuum systems find frequent use in the manufacture of semiconductor devices, for example, in etching processes or in high-density plasma chemical vapor deposition processes. In these processes, there are process gases introduced into the chamber that are subsequently pumped through the vacuum system (the TMP and valve assembly). There are several important aspects that are considered in the design of a vacuum system for use in processing including thermal management of the vacuum system, process control through-flow conductance variations, compatibility of the components with the process gases, the amount of total space physically occupied by the vacuum system and the proportions of this space, and the serviceability of the vacuum system.
Thermal management of the vacuum system is critical to some processes. In some processes, the vacuum lines, valves, and TMP are heated to a certain temperature to prevent both corrosion and condensation of the gases onto any of the surfaces in contact with the fluids. Any condensation will create not only solids (by definition), but also a source of particle impurity. Elements of the condensation can separate and find themselves within the gas stream being pumped. These particles can back-stream against the flow of the process gases and land onto the wafer substrate being processed, or other item within the chamber of interest. For semiconductor processing in particular, the separation between critical circuit components and connections on the wafer can be many times smaller than the particle that lands on the device, thus rendering the device on the wafer useless.
The mechanism of particle generation is a central point of focus in semiconductor processing. Despite the attention, the fundamental mechanism of particle generation is not fully understood. Nonetheless, temperature differentials, moving parts within the gas stream, and material composition all can exacerbate cleanliness issues.
In the vacuum systems used today there are temperature differentials created through the use of separate thermal management systems applied to the bypass line, the various valves, and the TMP. For example, the valve to the inlet of the pump, typically a gate valve, throttle-valve, or combination throttle and gate valve, is usually fitted with some sort of heater to raise the temperature of the components. The TMP will also be fitted with a heater to keep its internal components warm. It is quite common for these temperatures to be different, thus creating a temperature gradient. Moreover, the bypass valve and TMP exhaust valve are also heated. Again, the temperatures of the valves may not be the same, and they will be different from that of the TMP. These temperature gradients can exacerbate a particle formation problem.
Another key element to a vacuum system used for processing is having a method of process control. This is normally accomplished by using a valve to the inlet of the TMP. The inlet valve, or valves, to the TMP typically perform two functions, isolation and variation of the flow conductance. Such inlet valves are referred to as gate valves or throttling valves depending upon their function. These functions can be performed by either one valve or by two separate valves. It is increasingly common to use a single valve to perform both functions. The inlet valve, a separate but necessary component to the vacuum system, can be of various types, one such type is a pendulum-type. This separate valve is connected to the inlet of the TMP through a vacuum seal and a means of clamping, such as bolts. The valve itself is connected to the vacuum-processing chamber through a similar interface.
The valve body itself serves various functions. One function is to support the weight of the TMP through connection to the chamber. Another function is to provide a vacuum seal to both the TMP and chamber, which entails precise machining and the use of special-material vacuum seals. A third function is to have sufficient strength to withstand the torque that can be generated in the event of catastrophic rotor destruction.
A further important element to a vacuum system used for processing is that of selecting the correct components that will comprise the vacuum system. Subtle discrepancies in component specification can result in premature failure of the system. For example, the incorrect use of a single vacuum seal with the wrong material in a fluorine-based process can cause a leak of the process gas, that is typically toxic or corrosive, and therefore cause a risk to health. Moreover, for reliability purposes, it is advantageous to reduce the number of seals used if at all possible to reduce the chance of incorrect application and design. The burden of selecting the correct components and methods of assembly lies with the design engineer. Due to the number of components, the engineering task can be complicated and time-consuming.
Yet another key element to a vacuum system used for processing is that of conserving the amount of space used by the vacuum system. In all processes, it is economically beneficial to reduce the amount of space occupied by a vacuum processing tool and the space occupied by the ancillary equipment required to make the process work well. In semiconductor processing applications, for example, the amount of space under the process tool, where the high-vacuum system is normally arranged (or at least a portion of such), is precious due to the large amount of equipment whose performance could benefit by being closer to the processing chamber. In vacuum systems, the amount of “footprint” space, the area consumed by the equipment from a top-down projection, is important. For example, it is important to arrange the vacuum valves in the vacuum system to avoid obstructions with other nearby equipment. It is also desirable to keep the bypass line as close to the TMP as possible to minimize the footprint.
Still another key element to a vacuum system used for processing is minimizing the cost and time of repair and service, thus maximizing the amount of available operating time. It is also important to minimize the amount of time required to interchange faulty components (or assemblies) with new ones. Today's TMP-based vacuum systems, comprising of a number of components, require a large amount of components to be held in stock for repairs and service. It is also advantageous to have vacuum systems comprise as few components as possible to minimize the amount of stock held for service repairs and replacements.